Hybrid deposition of thin film solid oxide fuel cells and electrolyzers

ABSTRACT

The use of vapor deposition techniques enables synthesis of the basic components of a solid oxide fuel cell (SOFC); namely, the electrolyte layer, the two electrodes, and the electrolyte-electrode interfaces. Such vapor deposition techniques provide solutions to each of the three critical steps of material synthesis to produce a thin film solid oxide fuel cell (TFSOFC). The electrolyte is formed by reactive deposition of essentially any ion conducting oxide, such as defect free, yttria stabilized zirconia (YSZ) by planar magnetron sputtering. The electrodes are formed from ceramic powders sputter coated with an appropriate metal and sintered to a porous compact. The electrolyte-electrode interface is formed by chemical vapor deposition of zirconia compounds onto the porous electrodes to provide a dense, smooth surface on which to continue the growth of the defect-free electrolyte, whereby a single fuel cell or multiple cells may be fabricated.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

The present invention relates to solid oxide fuel cells, particularly tothin film solid oxide fuel cells, and more particularly to thefabrication of thin film solid oxide fuel cells using vapor depositiontechniques to miniaturize the assembly thereof for the formation ofsingle and multiple cells.

Fuel cells are electrochemical devices that convert the chemical energyin hydrogen or carbon monoxide and oxygen (in air) to electricity. Asolid oxide fuel cell (SOFC) consists of three basic components: anelectrolyte separating an anode and cathode. A thin film solid oxidefuel cell (TFSOFC) offers improvements in cost, reliability, efficiency,power density and specific power over other fuel cells.

An option to eliminate emissions from vehicles, reducing dependence onoil and enabling the use of alternative fuels, is the development andcommercialization of fuel cells that are cost effective, safe, andreliable. The maturation of vacuum coating technology in themicroelectronics and photovoltaics industries has enabled the potentialmanufacture of thin film solid fuel cells (TFSOFCS) at costs less thanany other current fuel cell design and possibly as low as the unit costof internal combustion engines. Whereas current SOFC designs, based onstate-of-the-art ceramic powder engineering require a 1000° C.operation, TFSOFCs can operate at temperatures less than 750° C. with adramatically reduced volume and mass for a given power output.

The reduction in operation temperature enables the alternative selectionof materials for the greatly reduced dimensions of thin films as opposedto the SOFCs synthesized with bulk ceramic methods. Herein lie thedifficulties encountered in TFSOFC synthesis and original solutions tothe following problems.

The electrolyte must have sufficient mechanical integrity to survivefabrication and operational environments. The electrolyte must be athermodynamically stable oxide layer on the order of 1-10 μm thick witha good oxygen ion transference number, low electron conductivity, andhigh enough density to prevent the fuel and air mixing. Yttriastabilized zirconia (YSZ) and cerium oxide (CeO₂), for example, arecandidate materials, but they have not been synthesized in sub 10 μmthick, defect-free layers. Essentially, any other ion conducting oxidescan be used.

The electrodes must be mechanically strong with a coefficient of thermalexpansion (CTE) that matches the electrolyte and must be electricallyconductive, yet sufficiently porous to allow for gas flow there through.Prior attempts to vapor synthesize a metal doped, YSZ layer which isporous have been unsuccessful. In addition, the electrolyte-electrodeinterface must be structurally stable and supply sufficient line contactfor the dissociation-recombination reactions as well as ionizations andion penetrations-extractions.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a thin film solidoxide fuel cell.

A further object of the invention is to provide thin film solid oxidefuel cell electrodes, and thin film solid oxide electrolyzers usinghybrid deposition techniques.

A further object of the invention is to utilize hybrid depositiontechniques for the formation of an electrolyte, electrodes, andelectrolyte-electrode interfaces for thin film solid oxide fuel cellsand electrolyzers.

Another object of the invention is to utilize reactive magnetrondeposition of ion conducting oxides, such as defect free yttriastabilized zirconia films, and defect free gadolinium doped cerium oxidefilms, of less than 10 μm thickness, for example, for use aselectrolytes.

Another object of the invention is to magnetron sputter coat ceramicpowders with an appropriate metal and sinter to a porous compact for useas anodes and cathodes.

Another object of the invention is to utilize chemical vapor depositionof zirconia compounds onto porous ceramic members to provide a dense,smooth surface on which to enable an effective electrolyte-electrodeinterface for single or multiple fuel cells.

Another object of the invention is to provide a controlled change inchemical vapor deposition flow parameters which enables high-ratedeposition of alternate porous and dense yttria stabilized zirconia(with in-situ metal infiltration into the porous layers to formelectrodes) resulting in the stacking of solid oxide fuel cell unitsthrough a continuous process.

Other objects and advantages of the present invention will becomeapparent from the following description and accompanying drawings.Basically, the invention involves a thin film solid oxide fuel cell(TFSOFC) produced by hybrid deposition techniques utilizing physicalvapor deposition (PVD), such as magnetron sputtering, and chemical vapordeposition (CVD) to produce the electrolyte and electrodes, and theelectrolyte-electrode interface. In addition, the invention involvescontrolling the CVD flow parameter to produce porous or dense layers,and infiltration of metal into the porous layers, to provide stacking offuel cell units through a continuous process. The use of vapordeposition techniques enables synthesis of the basic components of asolid oxide fuel cell (SOFC), which are 1) the electrolyte layer, 2) thetwo electrodes, and 3) the electrolyte-electrode interfaces.

The fabrication process involves synthesis of an electrolyte layer lessthan 10 μm thick of yttria stabilized zirconia (YSZ) for low temperatureoperation (<750° C.). The YSZ electrolyte structure is crystalline aswell as dense and defect free, and formed by reactive magnetrondeposition. The electrodes consist of a porous matrix which iselectrically conductive and composed, for example, of metal coated YSZpowder, formed by magnetron sputtering, and thereafter sintered to forma porous compact with continuous porosity, e.g with 55-25% porosity. Theelectrolyte-electrode interface is formed by chemical or physical vapordeposition of zirconia compound onto the porous electrodes to form asmooth dense surface on the electrode surfaces. The electrolyte andelectrodes may be solid-state bonded together or formed in a continuousmanner by control of the PVD and CVD techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate an embodiment of the invention and methodsof fabrication and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a cross-sectional view of an embodiment of a thin film solidoxide fuel cell made in accordance with the invention.

FIGS. 2A-2C illustrate a fabrication technique for producing the fuelcell of FIG. 1.

FIGS. 3A-3D illustrate another fabrication technique for forming to fuelcell of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The invention involves hybrid deposition of thin film solid oxide fuel,electrodes, cells and electrolyzers. A solid oxide fuel cell (SOFC)consists of three basic components: an electrolyte separating an anodeand a cathode. Vapor deposition techniques will independently anduniquely provide solutions to each of the three critical components bymaterials synthesis needed to produce a thin film solid fuel cell(TFSOFC). Advantageous use of thin film deposition technology serves tominiaturize the assembly and reduce the cost of SOFCs, therebyrevolutionizing potential applications. The invention involves threebasic operations: 1) formation of an electrolyte, 2) formation ofelectrodes, and 3) formation of electrolyte-electrode interfaces. In afirst technique, reactive deposition of defect free yttria stabilizedzirconia (YSZ) films less than 10 μm thick for use as electrolytes isaccomplished through the reactive sputtering of yttrium-zirconium alloytargets with planar magnetrons. In a second technique, ceramic powdersare sputter coated with an appropriate metal, compacted and sinteredinto a porous compact in a plate geometry for use as anodes andcathodes. Alternatively, the electrodes are formed by a co-deposition ofsputtering the metal appropriate for the anode (or cathode) withsputtering of an oxide material (e.g. zirconia) whose coefficient ofthermal expansion (CTE) matches the electrolyte layer (e.g.yttria-stabilized zirconia). In a third technique, chemical vapordeposition of zirconia compounds onto the porous ceramic compactsprovides a dense, smooth surface for bonding to the electrolyte, or onwhich to continue the growth of the defect free electrolyte for stackingof SOFC units through a continuous process. The electrolyte-electrodeinterfaces must be structurally stable and supply sufficient linecontact for the dissociation-recombination reactions as well asionizations and ion penetrations-extractions.

As pointed out above, the invention involves the use of vapor depositiontechniques which enable synthesis of the basic components of a SOFC;namely, the electrolyte layer, the two electrodes, and theelectrolyte-electrode interfaces. As described in detail hereinafter,the first two of the above-identified components are produced usingphysical vapor deposition (PVD) with planar magnetrons, and the last oneof the above-identified components is produced using chemical vapordeposition (CVD). Thus, the present invention involves hybrid depositionof thin films for producing solid oxide fuel cells, electrodes andelectrolyzers, and which overcome the problems associated with priorefforts to produce TFSOFCs having a thermodynamically stable electrolyteof a thickness of less than 10 μm; electrodes which are mechanicallystrong, electrically conductive, and with a coefficient of thermalexpansion that matches the electrolyte, yet porous enough to allow forgas flow; and an electrolyte-electrode interface that is structurallystable and sufficiently smooth to enable the reactions and ionizationswhich occur.

Synthesis of an electrolyte layer less than 10 μm thick of yttriastabilized zirconia (YSZ) is necessary for the low temperature operation(<750° C.) of a TFSOFC. The structure of the YSZ layer is crystalline aswell as dense and defect free, i.e. there are no continuous voidsthrough the film thickness. This is accomplished by the use of reactivedeposition of a metal alloy target of ZrY (˜85 at. % Zr) onto atemperature controlled substrate (temperature of 600° to 750° C.).

The electrodes consist of a porous matrix which is electricallyconductive. Utilizing one fabrication technique of the presentinvention, YSZ powder (˜1 μm size) is sputter coated with metal (Ni forthe anode and Ag for the cathode) then compacted and sintered to form a50% porous compact in a plane geometry (<1.5 mm thick). This procedurehas proven successful and leads to significantly less expensive thincathodes than the currently used strontium-doped lanthanum manganite.More generally, porosity ranging from about 55% to about 25% is desired.The upper limit for porosity is dictated by structural strengthrequirements while the lower limit is the result of the inability toform less porous bodies.

Alternatively, growth of either the anode or cathode can continue fromdeposition of the electrolyte layer by co-sputtering the appropriatemetal, e.g. Ni for the anode and Ag for the cathode. In this way, a unitcell can be vapor deposited in a continuous process. For both the coatedpowder and continuous deposition methods of electrode synthesis, themetal content of the electrode is less than 15% by volume. The low metalcontent (as compared to 30-50% as used in bulk cermets) provide acoefficient of thermal expansion for the electrode that matches theelectrolyte layer ensuring a composite structure which is thermallystable and not as susceptible as the bulk cermet electrode-based fuelcell to failure by thermal shocking and cycling.

Vapor deposition of zirconia onto a porous substrate is pursued tocreate a smooth dense surface on the electrode layer on which tocontinue deposition of the YSZ electrolyte layer either through chemicalvapor deposition (CVD) or physical vapor deposition (PVD). The viscousflow of the CVD process (at higher pressures than that conventionallyused for sputtering) uniquely enables near surface filling of the porouscompact to a dense overlayer. A controlled change in the CVD flowparameters enables high-rate deposition of alternate porous and denseYSZ layers (with in-situ metal infiltration into the porous layers toform electrodes) resulting in the stacking of SOFC units, also through acontinuous process.

Upon successful completion of the above described three tasks oroperations, the assembly of a TFSOFC is approached in the following twoways. The first is to bond at elevated temperature the YSZ electrolyteto the porous compact electrodes. Since reaction rates are high atoperating temperature, catalysis of fuel and air between the electrodeand electrolyte is limited to the interface region of linear contact.This assembly process is carried out to produce the embodiment ofFIG. 1. The second method of cell assembly (similar to that describedabove relative to CVD process) begins by utilizing the CVD infiltrationof the 1-2 μm top region of the porous electrode to increase the area ofcontact to the overgrowth YSZ electrolyte layer for catalysis of fueland air, and whereafter the other electrode is formed.

As seen from the above general description of the invention the TFSOFCscan be fabricated and/or assembled using two different approaches, eachinvolving hybrid deposition techniques. An embodiment of a TFSOFCproduced by PVD is illustrated in FIG. 1, and comprises a layer ofelectrolyte 10 having on opposite sides electrodes 11 and 12 whichinclude thin electrolyte-electrode interface layers 13 and 14. Theopposite sides of the electrode 10 are shown interconnected via anelectrical load, e.g. circuit 15 including a resistor 15', with an arrow16 indicating oxygen (air) an arrow 17 indicating hydrogen (fuel), andan arrow 18 indicating water. The electrolyte 10 has a thickness of lessthan 10 μm and composed of crystalline YSZ, with electrode 11 having athickness of up to 500 μm and composed of Ag coated YSZ power, withelectrode 12 having a thickness of up to 500 μm and composed of Nicoated YSZ powder, and interface layers 13 and 14 having a thickness of1-2 cm and composed of YSZ or ZrO₂. The electrolyte ionic behavior andphysical integrity can be enhanced by forming the structure as amultilayer process.

The following sets forth a detailed operational sequence for fabricationand assembly of the embodiment illustrated in the drawing, which iscarried out as follows:

The electrolyte layer (10) can be formed using sputtering deposition.For the use of yttria-stabilized zirconia as the electrolyte, both rfsputtering from a stoichiometric target and reactive sputtering of azirconium-yttrium metal alloy target are used.

1. The physical vapor deposition of 1-10 μm thick films ofyttria-stabilized zirconia (YSZ) is approached through 13.56 MHzrf-sputtering a target of (Y₂ O₃)₆ (ZrO₂)₉₄. The deposition chamber iscryogenically pumped from atmospheric pressure to a base pressure of<6×10⁻⁶ Pa in 12 hrs. including a 4 hr., 100° C. bake out. Room (20°-30°C.) to elevated (<750° C.) substrates (i.e. the electrodes) arehorizontally positioned 3-10 cm away from the center of the YSZ target.The Ar sputter gas pressure is regulated from 0.4 to 5 Pa (3 to 38mTorr) at a constant flow rate. The sputter gas pressure used toinitiate the YSZ deposit is selected to maximize contact area over theporous substrate surface. Low sputter gas pressures (<2 Pa) favorenergetic sputtered neutrals and line-of-sight deposition. High sputtergas pressures (>5 Pa) favor low surface mobility and very highscattering. Therefore, to promote the filling of voids in the poroussubstrate, the YSZ is initially sputtered at a gas pressure of 3-5 Pa(22-38 mTorr) to form either the electrolyte-cathode interfacial layer(13) for the case of deposition onto a cathode substrate or theelectrolyte-cathode interfacial layer (14) for the case of depositiononto an anode substrate. The gas pressure is then reduced to promote adense film growth in the bulk of the electrolyte layer (10). Theelectrolyte film is grown to be free of pinhole defects that would allowan uncontrolled mixing of the oxidant (16) with the fuel (17). A postdeposition air anneal of the electrolyte film can be performed tostabilize the desired cubic crystallographic phase in the sputtereddeposit, e.g, the zirconia films are heated in air to 700°±3° C. at arate of 15°-17° C. min.⁻¹, held at temperature for 1 hr. then cooled toroom temperature.

2. Yttria-stabilized zirconia (YSZ) films are synthesized using reactivedc magnetron sputter deposition. A homogeneous alloy of Zr--Y isprocessed into a planar magnetron target by rolling an alloy button. TheZr--Y button is first formed by electron beam arc melting a total massratioed as 489 gms of Zr to 86 gms of Y in order to produce an alloycomposition of 15 atomic percent Y. When appropriately oxidized, theresulting yttria-stabilized zirconia stoichiometry is (Y₂ O₃)₀.02(ZrO₂)₀.92. This 8% yttria composition (as compared to 5-6% asconventionally used in oxide target synthesis) yields the optimum oxygenion conductivity for cubic YSZ at elevated temperatures. The depositionchamber is cryogenically pumped from atmospheric pressure to a basepressure of <6×10⁻⁶ Pa in 12 hrs. which includes a vacuum bake-out at250° C. for 2-4 hours. The Zr--Y target is sputtered with anArgon-Oxygen gas mixture to reactively form 0.5-10 μm thick Zr--Y--Ofilms. The deposition parameters of current density, substratetemperature, working gas pressure and flow are determined in order toproduce the cubic YSZ phase at high rate. High rate deposition isrelative to process conditions where the target sputters in ametallic-mode as opposed to the low yield oxide-mode. At a target power(P) of 170 Watts and gas pressure (p) of 0.67 Pa (5 mTorr), a depositionrate hysteresis with gas flow (q) is determined in order to define aspecific flow for which both the oxide- and metallic- modes of targetsputtering can be achieved. Selection of a specific flow will then allowfor a near continuous variation of oxygen content in the forming film asa function of target power, i.e. current density which is proportionalto the sputter deposition rate. The variation of oxygen composition inthe deposited film as a function of deposition rate has been shown forthe reactive sputtering of Mo and Y metal targets. The initialhysteresis deposition experiment leads to the selection of a nominal gasflow with which to then conduct further deposition experiments toevaluate the dependency of oxygen content on deposition rate. Thesubstrate temperature is controlled and varied by inductive heating to700° C., the nominal operating temperature of a thin film YSZ-based fuelcell. The variation of gas pressure to initially fill surface voids thenstabilize a pinhole free growth of the electrolyte layer follows asdescribed for the rf sputter deposition process that utilizes astoichiometric YSZ target composition.

3. The use of compositionally different electrode-electrolyteinterfacial layers (13 and 14) can also be added to enhance performanceof the unit fuel cell by reducing interfacial reaction resistancesduring the generation of electrical current at operating potential (15).For example, it is common knowledge that Y-stabilized Bi₂ O₃ can be usedat the cathode side and Y-doped CeO₂ can be used at the anode side.

The electrodes (11 and 12) may also be synthesized through compaction ofmetal coated powders, as described hereinafter.

4. Stabilized zirconia powder is metal coated, with Ag for the cathode(11) and Ni for the anode (12), then compacted and sintered (oralternatively, hot pressed) to form porous electrode wafers which willserve as platforms (i.e. substrates) for the electrolyte depositionpreviously described and for ensuing cell fabrication. The electrodewafers are composites which consist of metal-coated powder that provideboth electrical conductance and whose controlled porosity enables thepassage of either fuel for the anode (12) or air for the cathode (11).This approach potentially eliminates the need for conductive poroussupports. Conventionally, appropriate metals are sintered with YSZpowder to form conductive but brittle electrodes which then requirestructural support as well as fuel/air flow passages. In addition, thecomposition of the conventional electrodes usually requires so muchmetal (>20% by volume) that the electrodes are no longer coefficientthermal expansion (CTE)-matched with the electrolyte layer. The methodof this invention of electrode synthesis minimizes the use ofmetallization in the anode and cathode to form a conductive matrix whichencases the substantive mass consisting of stabilized-zircoria toCTE-match the electrolyte layer. Stabilized-zirconia powder has beencoated with Ni using an intermediate metallization layer to ensureadhesion when thermal cycled. The deposition chamber is cryogenicallypumped from atmospheric pressure to a base pressure of <5×10⁻⁶ Pa in 12hrs. which includes a vacuum bake-out. Zirconia or YSZ powders of <50 μmin size are first subjected to a high pressure (>6 Pa) plasma ashingprocess with He gas that remove hydrocarbons from the particle surface.The powder particles are next sputtered coated at ambient temperature.Initially, an adhesion layer of Cr metal and/or Ti metal that can be<0.1 to >0.31 μm thick is applied. The powders are subjected to avibrational load during sputter coating which ensures random tumblinghence uniform coating of the surface for each powder particle. In asequence of final powder metallizations, Ni is the coating for use as ananode whereas Ag is the coating for use as the cathode. The electrodemetal thickness required is only 0.5-1 μm. The metal coated powders arecold pressed into wafers with compressive stresses of 10-40 ksi and thensintered for bonding at elevated temperatures (up to 1100° C.). Thethermal history in the electrode processing ensures stability for use atnominal fuel cell operating temperatures of 700°-800° C.. Wafer forms ofthese electrodes have been produced as examples with diameters from 6 to25 mm and thicknesses of 0.4 to 0.5 mm. The metal composes less than 10%of these electrodes by volume ensuring excellent thermal expansion andcontraction behavior during temperature cycling.

As pointed out above, the TFSOFCs can be produced utilizing acontinuation hybrid deposition process, and the following sets forth aspecific example of such.

Several paths are used to form TFSOFCs. An example can be givenutilizing the details of each process step as previously described. Acontinuous vapor deposition process utilizes multiple sources and atemperature controlled substrate platen. A source is provided for eachconstituent layer compound or to form each compound layer from elementaltarget/reactor materials depending on whether a PVD process assputtering or a CVD process step is used. For simplicity, an example isgiven using solely magnetron sputtering, with reference to FIGS. 2a, 2band 2c.

1. To start with, an anode 12 is formed, see FIG. 2a, wherein Ni and YSZare co-sputtered from separate sources at elevated gas pressure to forma porous layer which contains a continuous matrix of metal. A removablebase layer, not shown, is used for initiation of the deposition process.

2. An anode interfacial layer, similar to 14 in FIG. 1, is formed byhalting the deposition of Ni and continuing with an elevated gaspressure deposition of YSZ and/or Y-doped CeO₂.

3. An electrolyte layer 10, as shown in FIG. 2b, is formed by continuingthe sputter deposition of only YSZ at a low gas pressure.

4. The deposition process continues in formation of a cathodeinterfacial layer, similar to 13 in FIG. 1, by optional deposition ofY-stabilized Bi₂ O₃.

5. The formation of a cathode layer 11 as shown in FIG. 2c, thencontinues by co-sputter deposition from Ag and YSZ sources at elevatedgas pressure to form the final porous layer.

The unit fuel cell built in steps 1-5 can be accomplished in a systemcomposed of a stationary substrate with alternating deposition cycles orby passing a moving substrate through a deposition system withsequential reaction zones.

6. Alternatively, spacers and interconnect layers, such as Inconel, canbe processed onto the unit cell sheet built in steps 1-5 usingconventional lithographic patterning, deposition, and etchingprocedures.

7. A continuous sheet of unit fuel cell can then be cut and assembledinto a fuel cell stack with processes used for conventional tapecalendaring.

The step 7 option provides for maximum use of TFSOFC throughout theentire stack assembly by repeating steps 1-6. This option provides themaximum power density using TFSOFC technology.

8. Alternative paths to assembly of a unit cell described in steps 1-5include, as shown in FIGS. 3a-3d, the use of metal coated powders asanode (12), FIGS. 3a and 3b and cathode (11), FIG. 3c, substrates onwhich to deposit interfacial layers, (14' and 13', respectively) andelectrolyte layers 10' and 11", FIGS. 3b and 3c.

9. The electrolyte-electrode pair 10' and 10" can then be joined to itsmate electrode by a sintering process to form a unit cell, as shown inFIG. 3d. This process produces a unit fuel cell taking advantage of thethinnest electrolyte layer and an intermediate reduction in electrodelayer thickness as compared to bulk cermet processing. The electrolytelayer has a thickness of less than 10 μm and the electrodes may have athickness of less than 750 μm.

10. Alternatively, CVD processing can be used for each sputteringprocess described above.

While the above embodiments have been described using YSZ or ZrO₂,essentially any ion conducting oxides, including CeO₂, can be used asthe electrolyte. Also, the cathode, in addition to Ag may be composed ofalternate noble metals, such as Pt or Pd, for example, or alternativecompounds, such as lathanum-strontium-manganate, La(Sr)Mn₃, orLa--Sr--Co--Fe alloys, for example, and the anode, in addition to Ni maybe composed of alternative hydrogen compatible transition metals, suchas cobalt, iron and Co--Fe alloys, for example.

It has thus been shown that the present invention provides for thefabrication and assembly of thin film solid oxide fuel cells andelectrolyzers using hybrid deposition. By the utilization of magnetronsputtering a chemical vapor deposition technique, each of the threebasic components of a TFSOFC are economically and effectively producedand assembled, while enabling operation of the TFSOFC at temperaturesless than 750° C..

While the above embodiments have been set forth for low temperatureapplications, less than 750° C., the TFSOFC of FIG. 1 can be utilizedfor high temperature applications by changing the composition of thematerials used. Where high temperature use is desired, the selection ofthe anode and the cathode metals must be chemically stable, as forexample, using Pt.

While particular operational sequences, materials, temperatures,parameters, and a particular embodiment has been described and orillustrated, such is not intended to be limiting. Modifications andchanges may become apparent to those skilled in the art, and it isintended that the invention be limited only by the scope of the appendedclaims.

The invention claimed is:
 1. A solid oxide fuel cell comprising:anelectrolyte layer consisting of an ion conducting oxide; a first porouselectrode positioned adjacent one surface of said electrolyte layer andconsisting of material selected from the group consisting of yttriastabilized zirconia and other material that matches the coefficient ofthermal expansion (CTE) of the electrolyte, coated with a materialselected from the group consisting of silver, platinum or palladium; asecond porous electrode positioned adjacent another surface of saidelectrolyte layer and consisting of material selected from the groupconsisting of yttria stabilized zirconia and other material that matchesthe CTE of the electrolyte, coated with a material selected from thegroup consisting of nickel, iron, cobalt, and Co--Fe alloys; and saidporous electrodes having surfaces defining electrolyte-electrodeinterfaces located intermediate the electrolyte layer and said porouselectrodes and consisting of material selected from the group consistingof yttria stabilized zirconia, and CeO.sub..
 2. The fuel cell of claim2, wherein the electrolyte-electrode interfaces are formed in a 1-2 μmthick region of the surface of said electrodes.
 3. The fuel cell ofclaim 1, wherein said electrolyte layer has a thickness of less than 10μm.
 4. The fuel cell of claim 1, wherein said electrodes have athickness in the range of 1 to 750 μm.
 5. The fuel cell of claim 1,wherein said electrolyte layer consists of crystalline yttria stabilizedzirconia, wherein said first porous electrode consists of silver coatedyttria stabilized zirconia, wherein said second porous electrodeconsists of nickel coated yttria stabilized zirconia, and wherein saidinterfaces consist of a zirconia compound.
 6. The fuel cell of claim 1;wherein said first and second porous comprise a 55%-25% porous compactformed from powder coated with about 1 μm thick metal.
 7. The fuel cellof claim 1, wherein said electrolyte-electrode interfaces define smoothsurfaces on said first and second electrodes.
 8. The fuel cell of claim1, wherein said first and second porous electrodes are bonded to saidelectrolyte layer via said interfaces.
 9. The fuel cell of claim 1,wherein said first and second porous electrodes are interconnected by asingle layer of electrolyte.
 10. The fuel cell of claim 1, wherein saidporous electrodes have a metal content of less than 15% by volume. 11.The fuel cell of claim 1, wherein said electrolyte layer , said porouselectrodes, and said electrolyte-electrode interfaces are constructed bya continuous deposition process.
 12. A method for producing solid oxidefuel cells using continuous deposition, comprising:forming an electrodehaving about 55%-25% porosity; forming an electrode-electrolyteinterface on at least one surface of the porous electrode; forming anelectrolyte on the interface; forming an interface on the electrolyte;and forming an electrode on the interface.
 13. The method of claim 12,wherein the sequence of operations are repeated to form a stacked solidoxide fuel cell.
 14. The fuel cell of claim 13, wherein the sequence offorming operations is carried out using continuous deposition.
 15. Asolid oxide fuel cell produced by a continuous deposition process,comprising:forming a first porous electrode; forming a firstelectrode-electrolyte interface on a surface of the first porouselectrode; forming an electrolyte on the first interface; forming asecond electrode-electrolyte interface on the electrolyte; and forming asecond porous electrode on the second interface.
 16. The solid oxidefuel cell of claim 15, wherein the continuous deposition processadditionally includes:forming another electrode-electrolyte interface onone or both of said porous electrodes; forming an electrolyte layer onone or both of the thus formed another interface; and forming anelectrode-electrolyte interface on one or both electrolyte layers; andforming a porous electrode on one or both of the thus formedelectrode-electrolyte interfaces.
 17. The solid oxide fuel cell of claim16, wherein the continuous deposition process additionally includesrepeating the sequence of forming operations to produce a stacked solidoxide fuel cell.
 18. Belt The solid oxide fuel cell of claim 15, whereinthe forming of the porous electrodes is carried out such that the porouselectrodes are formed with a porosity of about 25-55%.
 19. The solidoxide fuel cell of claim 15, wherein the forming of the porouselectrodes is carried out to produce the electrodes with a metal contentof less than 15% by volume.